1. Field of the Invention
The present invention described in this patent specification relates to an organic EL (Electro Luminescence) display panel driven/controlled by adoption of an active matrix driving system and relates to a driving technology for driving the organic EL display panel. It is to be noted that the present invention described in this patent specification has three modes, i.e., an organic EL display panel, an electronic apparatus employing the organic EL display panel and a method for driving the organic EL display panel.
2. Description of the Related Art
FIG. 1 is a general circuit block diagram showing an organic EL display panel 1 driven/controlled by adoption of an active matrix driving method. As shown in the circuit block diagram of FIG. 1, the organic EL display panel 1 employs a pixel array section 3, a signal-write control line driving section 5 and a horizontal selector 7. It is to be noted that the pixel array section 3 includes pixel circuits 9 each located at an intersection of a signal line DTL and a write control line WSL.
Incidentally, an organic EL device employed in each of the pixel circuits 9 is a light emitting device which emits light in accordance with a current flowing thereto. Thus, the organic EL display panel 1 adopts a driving method for controlling gradations of pixels by adjustment of a current flowing through the organic EL device. FIG. 2 is a block diagram showing a simplest circuit configuration of a pixel circuit 9 connected to the horizontal selector 7 by a signal line DTL and the signal-write control line driving section 5 by a write control line WSL. As shown in the block diagram of FIG. 2, the pixel circuit 9 includes a sampling transistor T1, a driving transistor T2 and a signal holding capacitor Cs in addition to the organic EL device OLED.
It is to be noted that the sampling transistor T1 is a TFT (Thin Film Transistor) for controlling an operation to store a signal electric potential Vsig corresponding to the gradation value of the pixel circuit 9 into the signal holding capacitor Cs. On the other hand, the driving transistor T2 is a thin-film transistor for supplying a driving current Ids to the organic EL device OLED on the basis of a gate-source voltage Vgs of the driving transistor T2, and the gate-source voltage Vgs of the driving transistor T2 is determined by the signal electric potential Vsig stored in the signal holding capacitor Cs. The driving current Ids is a current flowing between the drain and source electrodes of the driving transistor T2 whereas the gate-source voltage Vgs is a voltage appearing between the gate and source electrodes of the driving transistor T2. In the case of the pixel circuit 9 shown in the block diagram of FIG. 2, the sampling transistor T1 is a thin-film transistor of an N-channel type whereas the driving transistor T2 is a thin-film transistor of a P-channel type.
In the case of the pixel circuit 9 shown in the block diagram of FIG. 2, the source electrode of the driving transistor T2 is connected to a fixed power-supply electric potential Vcc by a current supply line which is also referred to as a power-supply line in this patent specification. The driving transistor T2 typically operates in a saturated region. That is to say, the driving transistor T2 functions as a constant-current source for supplying a driving current Ids having a magnitude determined by the signal electric potential Vsig to the organic EL device OLED. The driving current Ids is expressed by the following equation:Ids=k·μ·(Vgs−Vth)2/2
In the above equation, reference notation u denotes the mobility of majority carriers in the driving transistor T2 whereas reference notation Vth denotes the threshold voltage of the driving transistor T2. On the other hand, reference notation k denotes a coefficient represented by an expression (W/L)·Cox where reference notation W denotes a channel width of the driving transistor T2, reference notation L denotes a channel length of the driving transistor T2 and reference notation Cox denotes a gate capacitance per unit area of the driving transistor T2.
It is to be noted that the driving transistor T2 employed in the pixel circuit 9 with a configuration shown in the block diagram of FIG. 2 is known to exhibit a drain-voltage characteristic which changes due to a process of aging in accordance with changes shown in a diagram of FIG. 3 as changes in I-V characteristic which represents a relation between the driving current Ids mentioned above and a voltage applied between the anode and cathode electrodes of the organic EL device OLED as a relation changing with the lapse of time due to a process of aging. Since the gate-source voltage Vgs of the driving transistor T2 is held at a fixed level by the signal holding capacitor Cs, however, the magnitude of the driving current Ids supplied to the organic EL device OLED does not change, allowing the luminance of light emitted by the organic EL device OLED to be kept at a constant value.
Documents used in this patent specification to serve as documents relevant to the organic EL panel display adopting the active-matrix driving method are listed as follows: Japanese Patent Laid-open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, and 2004-093682.